The present invention relates to pressure vessels and, in particular, it concerns a pressure vessel which has a thin unstressed metallic liner.
A number of different structures are known for containing fluids at elevated pressures. These structures are generally referred to as "pressure vessels". Requirements of safety, as well as attempts to reduce weight, have lead away from the use of simple metallic pressure vessels towards use of reinforced composite materials. In order to provide the required sealing characteristics, however, an additional inner liner must be provided. Hence the two principal types of pressure vessel currently in use both employ reinforced composite containers with either a seamless metallic or thermoplastic liner.
The use of a metallic liner generally provides a much longer operational life, better resistance to harsh environments, and better sealing characteristics than thermoplastic liners. The design of pressure vessels with metallic liners, however, presents its own particular problems, as will now be described.
Composite pressure vessels with metallic liners are manufactured by filament winding of fibers impregnated with resin matrix, together forming the composite material, around the metallic liner. The metal liner of these structures bears part of the applied internal pressure. In addition, incompatibility of the ranges of elastic behavior of the metal liner and composite material lead to residual compression stresses in the liner as a result of the "proof pressure" test ("autofrittage phenomenon").
During subsequent application of internal pressure, the liner stretches and experiences corresponding tensile stress. In order to withstand these tension/compression stresses through repeated filling cycles over an extended period of usage, the liner must be relatively thick. Besides the clear implications of a thick liner for the weight of the vessel, the presence of a thick metallic layer also leads to safety problems.
In an effort to address these problems, attempts have been made to develop an unstressed metallic liner in which a thin metallic layer provides sealing properties while transferring all of the pressure load to the surrounding primary vessel. An example of such a structure is described in U.S. Pat. No. 5,292,207 to Lueke.
In order to avoid stressing of the liner, Lueke suggests a complicated "herringbone" pattern of parallelogram-like elements which provides undulations in two orthogonal directions. As a result, the liner readily stretches in any direction to conform to the deformation of the primary vessel.
The structure suggested by Lueke presents numerous problems of practical implementation. Firstly, the liner appears to contact the primary vessel at isolated points. Pressure applied to such a structure would not be effectively transferred to the primary vessel walls, and would probably result in immediate destruction of the herringbone pattern. Furthermore, the complicated structure would be extremely difficult to manufacture.
Another reference, U.S. Pat. No. 1,968,088 to Mekler, although less relevant than the Lueke reference, will be mentioned for its superficial similarity to one embodiment of the present invention. Mekler, in a patent filed before the introduction of reinforced composite materials into the art, describes a freely-expanding, corrugated protective liner for reaction vessels subjected to rapidly varying temperatures. The corrugations serve to prevent distortion and damage to the liner under extreme heat stress, while insulating the main vessel from the most extreme of the temperature variations. The reference does not address issues of performance under elevated pressure.
The structure described by Mekler is not suitable for use with fluids at elevated pressures. Since no solution is suggested for accommodating heat stress along the direction of elongation of the corrugations, it would appear that the liner must have a clearance from the ends of the primary vessel. As a result, the liner must be designed to bear a large proportion of any internal pressure. Additionally, no support is provided for the corrugations of the liner. Thus, if the liner was made from thin materials, the corrugated structure would rapidly deform and collapse under internal pressure. Finally, since this reference pre-dates the use of reinforced composite materials, Mekler clearly fails to teach any synergy between a liner structure and specific configurations of such composite materials.
There is therefore a need for pressure vessels with thin unstressed metallic liners which are convenient to produce and which effectively transfer applied pressure to the walls of the primary container.